The field of the invention is that of virus vaccines for protection of dogs against infection by canine parvovirus, their production and use.
Parvoviruses are characterized as small animal DNA viruses containing of an isometric protein capsid and a short molecule of single-stranded DNA. Although parvoviruses have been recovered and isolated from various animals, there had been no definite isolation of pathogenic canine parvovirus until recently (Siegl, The Parvoviruses, Springer-Verlag, New York 1976). Bachmann et al. include the dog as a possible parvovirus host in a report detailing the characteristics of parvoviruses in general (Bachmann et al., Intervirology 10: in press, 1978). In 1970, Binn et al. reported the recovery and characterization of a "minute virus of canines" (Binn et al., Infect. Immun. 1: 503, 1970). The isolates described were of canine origin, however, their pathogenicity was not known, and cytopathic effect (CPE) was produced in only a very narrow host range, i.e. only in a single continuous canine cell line, and not in primary canine nor primary or continuous cell cultures from other species. Pathogenicity for dogs was not determined nor was evaluation of vaccine potential done. Based on known properties of the Cornell isolates, it is clear that the recent CPV isolates are not the same as the "minute virus of canines" as described by Binn. In 1977, Eugster and Nairn reported a circumstantially-suggested causative link between diarrhea in puppies and a canine parvovirus. (Eugster, Nairn, Southwestern Veterinarian, 30: 59, 1977). The isolate reported therein could not be serially propagated in MDCK cells, the only cell line tested. Again, pathogenic potential was unexamined and no experimental animal inoculations were performed. In 1978, widespread outbreaks of an apparently new disease in canines appeared (Appel, Cooper, Greisen and Carmichael, JAVMA 173(11) 1516-1518; Dec. 1978), occurring in both the United States and Australia (unpublished). The natural disease is characterized by diarrhea, fever, and leukopenia (relative lymphopenia).
The first isolation of a distinct parvovirus, and its in vitro propagation in primary cells and in cell lines of various species such as mink lung cells and canine kidney was described by two of the inventors in 105 Veterinary Record 156, and in their U.S. Pat. No. 4,193,991, relating to a killed canine parvovirus vaccine.
The factors involved in selective genetic pressure on a virus were described in an article in 20 Infection and Immunity 108, April 1978, Carmichael & Medic, although that article dealt with a different virus (canine herpesvirus). A method of determining virulence through the use of plaque-size was disclosed in Inventor Carmichael's patent application Ser. No. 5,743 filed Jan. 23, 1979, "Small Plaque Variant Canine Herpesvirus Vaccine."
However, although heterotypic (Carmichael, et al, U.S. Pat. No. 4,193,990) and killed vaccines (Carmichael, et al, U.S. Pat. No. 4,193,991) have been known previously, until this invention there has been no successful production of a modified live vaccine for CPV. The production of a modified live CPV vaccine represents a novel and distinct advance in the art. A live virus vaccine constitutes a significant advance over heterotypic and killed vaccines because the magnitude of the immune response represents a 4- or 5-fold improvement over that produced by the other vaccines. The duration of immunity is vastly prolonged, and the live vaccine is commercially easier and cheaper to produce.